A common method for delivering a cement paste (or any other highly viscous paste) into a bone (or other tissue or organ of a diseased subject) cavity by a minimally invasive procedure is by pressing the paste into the cavity through a thin tube percutaneously inserted into the cavity. To fill in a bony cavity, PMMA and calcium-based cements are most commonly used. The cement paste is generally prepared by mixing a solid powder component with a liquid component (setting solution) at a proper ratio to form a paste. The mixed paste is set (hardened) and starts to gain its strength generally within minutes or tens of minutes, depending on the setting time of the formula being used.
To minimally invasively transport the cement paste into the cavity through a thin tube, conventionally, a syringe type device is used, wherein the mixed cement paste is stored in a container (reservoir) and pushed by a piston or a plunger through a small exit connected to a thin tube that has been percutaneously inserted into the bone cavity. Since the cross-sectional areas of the container and the exit are largely different (For example, the cross-sectional area of the container of a conventional 10 c.c. syringe is larger than that of its exit by about 60 times; and the cross-sectional area of the container of a conventional 20 c.c. syringe is larger than that of the exit by about 100 times), a large pressure is often implemented in order to push the cement out of the exit. However, larger pressures require more complicated designs in the delivery tool as well as higher costs. This problem becomes especially serious in delivering a highly viscous paste, such as PMMA and calcium-based cement. Although a more dilute paste prepared with a higher liquid/powder ratio may make the paste flow and be pushed through the small exit more easily, unfortunately, a dilute paste almost always leads to poor material properties. This is a big dilemma in this field. Theoretically, using a thin container (reservoir) with a diameter similar to that of the thin surgical tube (or connecting tube) may overcome the problem caused by the large difference in cross-sectional area between container and its exit. Practically, however, in so doing, an extremely long container is required. For example, to minimally invasively deliver a cement paste of 5 cc in volume through a container with an inner diameter of 1 mm would require a container of more than 6 meters in length; and to deliver a 10 cc cement paste through the same container would require a container longer than 12 meters! (A plunger of the same length is also required to drive the paste all the way to the container exit) This kind of length is practically impossible for any kind of surgery. Furthermore, even if an extremely long, thin container is used, it would be practically impossible to transport the mixed cement paste into this thin container. When a regular syringe is used, the large cross-sectional difference-induced problem still remains in the transportation of the paste from the syringe into the thin container.
Another primary problem with the conventional syringe-type cement deliverer is that a larger pressure does not guarantee a more efficient delivery. In many cases (for example, for most calcium-based cements), the opposite is true. This is because that, before being fully hardened due to the reaction between powder and liquid, the cement paste is still a solid-liquid two phase material. Under pressure, the liquid phase tends to separate from the solid phase. Since the greatest pressure gradient occurs at the exit (thin neck) region, the liquid tends to flow out of the exit at a higher speed than the solid. Due to this solid-liquid separation effect, the cement paste coming out of the container at the early stage has a higher-than-desired liquid/powder ratio, causing properties of the cement to degrade. On the other hand, the cement remaining in the container, especially at the later stage, has a lower-than-desired liquid/powder ratio (because the lower-density liquid continues to be squeezed out of the container) and becomes difficult to flow out of the exit.